Kaga's half-sister was
Akagi. Akagi and its sister ship
Amagi were laid down as battlecruisers but, because of the Washington Naval Treaty, were intended to be converted into carriers, just as USS
Lexington and USS
Saratoga were.
But
Amagi, under construction, was damaged beyond practical repair by the Tokyo earthquake of Sep 1923, and so the battleship-under-construction
Kaga was converted instead. There were two other
Amagi-class battlecruisers already laid down, but
Kaga was further along.
Perhaps a bad choice.
Kaga was a less-suitable basis for conversion. Based on a battleship rather than a battlecruiser, she was slower and less maneuverable than
Akagi. And the two carriers--making up the 1st division of the Kido Butai--did not have common performance or logistical characteristics.
As built, Kaga and Akagi had three superimposed flight decks.
KagaBoth were reconstructed in the mid-1930s to have a single flight deck.